1. Field of the Invention
The present invention relates to a method for determining of the amount of sulfur loading in organic polysulfides.
2. Description of the Prior Art
It is known that organic disulfides react with elemental sulfur in the presence of certain catalysts to form organic polysulfides. The average number of sulfur atoms contained in the organic polysulfides is referred to as the "sulfur rank" and is determined by the amount of sulfur actually incorporated into the sulfur-laden disulfide (i.e., the polysulfide).
Recently, catalyzed dimethyldisulfide (DMDS) has become the solvent of choice for removing or preventing the formation of sulfur plugs in sour-gas wells. In fact, in certain wells, solvent must be continuously injected in order to maintain gas production. Accordingly, economics dictate that the sulfur-laden DMDS (i.e., the dimethylpolysulfide, or DMPS) be recovered, and reinjected downhole. Removal of the sulfur from the DMPS is referred to as regeneration. The extent of the sulfur loading must be quantified in order to control and monitor the regeneration process and hence, the overall operation.
Many approaches to the analysis of diand polysulfides have been reported in the literature over the years. The Analytical Chemistry of Sulfur and Its Compounds, J.H. Karchmer, Vol. 29, Part II, Wiley, N.Y., pp. 322-431 (1972), provides an excellent review of these methods.
In Tetrahedron, 18, pp. 205-218 (1962) Moore and Trego reacted triphenylphosphine with dialkyl and diaryl tetrasulfides in benzene to obtain the corresponding disulfides. The reaction was followed by the iodometric titration of the unreacted triphenylphosphine to determine the amount of sulfur loading indirectly by titrating for the disappearance of the phosphine. While this provides the amount of sulfur loading, it does not allow the quantification of the amount of disulfide present in the sample.
The reaction of various phosphorous containing species with di- and polysulfides from the viewpoint of synthesis is known in the art. For example, see J. Am. Chem. Soc., 104, p. 6045 (1982).
High pressure liquid chromatography is useful for determining polysulfide distribution in a sample of well effluent, but is fairly complicated and not suitable for field application.
A method for determining the amount of elemental sulfur present in a material suspected of containing elemental sulfur has been reported by Lesage and Clark in Alberta Sulfur Research Ltd, Chalktalk, Jan. 26, 1988. This method provides for the determination of elemental sulfur by reacting a sample that contains elemental sulfur with triphenylphosphine followed by quantification of the formed triphenylphosphine sulfide by gas chromatography. However, this method does not disclose the determination of sulfur loading in organic polysulfides.
Other examples of the use of gas chromatography to analyze di- or trisulfides are known in the art, e.g., J. Pet. Res., 6 (2), 153-61 (1987); J. Chromatogr., 365, 205-212 (1986); J. Chromatogr. 319 (2) , 187-94 (1985); Am. Chem. Soc., Div. Pet. Chem., 28 (5), 1218-23 (1983); and Clin. Chim. Acta., 130 (1), 103-10 (1983).
However, analysis of polysulfides by gas chromatography has been generally limited to the analysis of polysulfides containing no more than five sulfur atoms due to the instability of higher polysulfide species toward these analytical procedures. That is, consistently accurate results in determining the sulfur rank or extent of sulfur loading in polysulfides cannot be obtained by using the prior art gas chromatographic analysis of polysulfides containing more than five sulfur atoms.
None of these references discloses a method for analysis of higher polysulfides to determine the amount of sulfur loading which may be conveniently used in the field to give consistent results.
The present invention provides an improved method for determining the extent of sulfur loading in an organic polysulfide, including higher polysulfides having more than five sulfur atoms. Moreover, the present invention allows for measurement of the total disulfide content of a given polysulfide sample, as well as of the content of impurities or additives, such as hydrocarbon solvents.